Add option to always format error messages

[Mark Wubben]

Concordance ignores non-enumerable properties. AVA uses Concordance to format caught errors. If a custom error is created with a non-enumerable message then this message is not included in the results.

IIRC we already handle name properties of errors, so we could include non-enumerable message properties by default (this would be a breaking change) or by configuration.

See also avajs/ava#2755.